/**
 * Created with Intellij IDEA.
 * Description:
 * User: 33013
 * Date: 2023-10-22
 * Time: 20:59
 */
import java.util.*;
public class Test {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        HashMap<Character, Integer> hash = new HashMap<>();
        String str = in.nextLine();
        int len = str.length();
        for(int i = 0; i < len; i++){
            char ch = str.charAt(i);
            hash.put(str.charAt(i), hash.getOrDefault(str.charAt(i) + 1, 1));
        }
        for(int i = 0; i < len; i++){
            if(hash.get(str.charAt(i)) == 1){
                System.out.println(str.charAt(i));
                return;
            }
        }
    }

    public static void main3(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        String str1 = in.nextLine();
        String str2 = in.nextLine();
        int max = 0;
        int l1 = str1.length();
        int l2 = str2.length();
        for(int i = 0; i < l1; i++){
            int x = i;
            int y = 0;
            int sum = 0;
            while(y < l2){
                while(y < l2 && x < l1 && str2.charAt(y) != str1.charAt(i)){
                    y++;
                }
                x = i;
                while(y < l2 && x < l1 && str2.charAt(y) == str1.charAt(x)){
                    y++;
                    x++;
                    sum++;
                }
                if(sum > max){
                    max = sum;
                }
            }
        }
        System.out.println(max);
    }

    public static void main2(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        while (in.hasNextInt()) { // 注意 while 处理多个 case
            String str = in.nextLine();
            StringBuilder str1 = new StringBuilder(str);
            str1.reverse();
            System.out.println(str1.toString());
        }
    }

    public static void main1(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        String str1 = in.nextLine();
        String str2 = in.nextLine();
        String str3 = div(str1, str2);
        System.out.println(str3);
    }
    private static String div(String str1, String str2){
        int len1 = str1.length();
        int len2 = str2.length();
        int start = 0;
        int end = 0;
        int max = 0;
        if(len1 > len2){
            for(int i = 0; i < len2; i++){
                int sum = 0;
                int j = 0;
                while(j < len1 && str1.charAt(j) != str2.charAt(i)){
                    j++;
                }
                if(j >= len1){
                    continue;
                }
                int z = j;
                while(j < len1 && i < len2 && str1.charAt(j) == str2.charAt(i)){
                    sum++;
                    i++;
                    j++;
                }
                if(sum > max){
                    start = z;
                    end = j;
                }
                if(j >= len1 || i >= len2){
                    end = j - 1;
                }
            }
            StringBuilder str3 = new StringBuilder();
            for(; start <= end; start++){
                str3.append(str1.charAt(start));
            }
            return str3.toString();
        }else{
            for(int i = 0; i < len1; i++){
                int sum = 0;
                int j = 0;
                while(j < len2 && str2.charAt(j) != str1.charAt(i)){
                    j++;
                }
                if(j >= len2){
                    continue;
                }
                int z = j;
                while(j < len2 && i < len1 && str2.charAt(j) == str1.charAt(i)){
                    sum++;
                    i++;
                    j++;
                }
                if(sum > max) {
                    max = sum;
                    start = z;
                    end = j - 1;
                }
                i--;
            }
            StringBuilder str3 = new StringBuilder();
            for(; start <= end; start++){
                str3.append(str2.charAt(start));
            }
            return str3.toString();
        }

    }

//    public static void main(String[] args) {
//        Scanner in = new Scanner(System.in);
//        // 注意 hasNext 和 hasNextLine 的区别
//        int sum = 0;//总共喝了多少，
//        int amount = 0;//空瓶子
//        while (in.hasNextInt()) { // 注意 while 处理多个 case
//            sum = 0;
//            amount = in.nextInt();
//            while(amount >= 3){
//                sum += amount / 3;
//
//                amount = amount % 3 + amount / 3;
//            }
//            System.out.println(sum);
//        }
//    }
}
